Method of reducing thermal distortion in grinding machines

ABSTRACT

The invention provides a method of reducing thermal distortion in grinding machines. Such machines each comprise a machine base ( 60 ) and a grinding wheel ( 50 ) for grinding components in the machine ( 10 ). The method includes the steps of: (a) sensing a first temperature at an upper surface of the base ( 60 ) substantially below a position ( 110 ) in the machine ( 10 ) whereat component grinding using the wheel ( 50 ) occurs; (b) sensing a second temperature of an underside surface of the base ( 60 ) substantially below the position ( 110 ) whereat component grinding occurs; (c) determining a relationship between component size drift and changes in a difference between the first and second temperatures; and thereafter (d) correcting a positional offset applied to the wheel ( 50 ) during grinding in accordance with the determined relationship, thereby reducing the component size drift. Preferably, the relationship is substantially a linear function of the form MDS=K f (ΔT 1 −ΔT 2 ) although higher-order polynomial correction can be applied if required. The invention also relates to grinding machines ( 10 ) employing the method of reducing thermal distortion.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of reducing thermaldistortion in grinding machines, such distortion resulting in size driftin components manufactured using the machines. The invention relates inparticular, but not exclusively, to a method of reducing thermaldistortion in grinding machines comprising associated machine bases overwhich a coolant flows in operation.

BACKGROUND TO THE INVENTION

[0002] It is generally known that components manufactured in grindingmachines comprising associated machine bases can suffer component sizedrift as a consequence of small movements and distortions in the bases.In a grinding machine comprising a base where a coolant fluid flows overthe base, it is a logical conclusion that distortions can at least tosome extent be affected by the temperature of the fluid.

[0003] The inventor has appreciated that size drift in componentsmanufactured in grinding machines employing associated base cooling canbe reduced by applying a simple correction method to the machines.

SUMMARY OF THE INVENTION

[0004] According to a first aspect of the present invention, there isprovided a method of reducing thermal distortion in a grinding machinecomprising a machine base and a grinding wheel for grinding componentsin the machine, the method including the steps of:

[0005] (a) sensing a first temperature at an upper surface of the basesubstantially below a position in the machine whereat component grindingusing the wheel occurs;

[0006] (b) sensing a second temperature of an underside surface of thebase substantially below the position whereat component grinding usingthe wheel occurs;

[0007] (c) determining a relationship between component size drift andchanges in a difference between the first and second temperatures; andthereafter

[0008] (d) correcting a positional offset applied to the wheel duringgrinding in accordance with the determined relationship, therebyreducing the component size drift.

[0009] The invention provides the advantage that the method is capableof reducing component size drift.

[0010] In practice, a coolant fluid is employed when grinding forcooling and removing grinding debris. The coolant fluid can besusceptible to transient temperature fluctuations. Thus, preferably, thebase includes a panel at its upper surface and the first temperature ismeasured within the panel away from an upper facing exterior surface ofthe panel susceptible to being exposed to the coolant fluid.

[0011] In order to ensure that the first temperature is relativelyunaffected by transient temperature fluctuations of the coolant fluid,it is preferable that the first temperature is measured at a distance ina range of 60% to 90% of the thickness of the panel away from the upperfacing exterior surface.

[0012] From detailed studies, the inventor has determined that the firsttemperature is beneficially measured in a coolant fluid gully includedin the machine below the position whereat grinding occurs, the gullybeing included for collecting coolant fluid output from the positionwhereat grinding occurs.

[0013] The inventor has found it convenient to measure the firsttemperature using a probe which is substantially thermally isolated fromany coolant fluid flowing over the upper surface of the base. Whendetermining a measurement position, the gully is most appropriate whenattempting thermal distortion correction.

[0014] The probe preferably provides an accurate indication of the firsttemperature. Thus, preferably, the first temperature is measured using aprobe which is in thermal communication with the upper surface of thebase by mediation of a heat conductive paste. The inventor has found itespecially convenient to employ a heat conductive paste comprisingsilicone.

[0015] Preferably, for convenience of installation and adjustment, thesecond temperature is measured using a probe magnetically attached tothe underside surface of the base.

[0016] During studies, the inventor has found it beneficial to achievingeffective thermal distortion correction to arrange for the positionwhereat component grinding occurs and the positions at which the firstand second temperatures are measured to be mutually substantiallyco-linear.

[0017] In a general situation, the inventor has found it beneficial toarrange for the relationship to be of the form

MSD=K _(f)(b ₁(ΔT ₁ −ΔT ₂)+b ₂(ΔT ₁ −ΔT ₂)²+ . . . +b_(n)(ΔT ₁ −ΔT₂)^(n))

[0018] wherein MSD is a grinding correction applied, K_(f), b₁ to b_(n)are proportionality constants, ΔT₁ is the first temperature, ΔT₂ is thesecond temperature, and n is a positive integer.

[0019] In practice, the inventor has found that the correction requireddoes not change abruptly with temperature. Thus, it is preferable thatthe relationship is a linear, quadratic or cubic function. Suchlower-order functions are relatively easy to cope with when calculatingtheir coefficients from test trials.

[0020] The inventor has found it especially appropriate to apply alinear correction for thermal distortion. Thus, preferably, therelationship is substantially a linear function of the form

MSD=K _(f)(ΔT ₁ −ΔT ₂)

[0021] Moreover, the inventor has evaluated from trials that, for onetype of grinding machine, the proportionality coefficient K_(f) ispreferably in a range of 25 to 35 μm/° C., namely the constant K_(f) isbeneficially substantially 30 μm/° C.

[0022] When determining the relationship, it is more complex tocharacterise the grinding machine when feedback is applied therearound.Thus, one or more of the proportionality constants are preferablycalculated empirically from open-loop trials undertaken on the machinewhere temperature correction derived from the base is not applied.

[0023] In order to achieve a satisfactory degree of correction, thefirst and second temperatures are measured to a resolution of at least0.1° C. More preferably, the first and second temperatures are measuredto a resolution of at least 0.05° C. If measurement technique allows, itis most preferable that the first and second temperatures are measuredto a resolution of at least 0.01° C.

[0024] According to a second aspect of the present invention, there isprovided a grinding machine employing the method according to the firstaspect of the invention, the machine comprising a machine base and agrinding wheel for grinding components in the machine, the machinefurther comprising:

[0025] (a) first temperature sensing means for sensing a firsttemperature at an upper surface of the base substantially below aposition in the machine whereat component grinding using the wheeloccurs;

[0026] (b) second temperature sensing means for sensing a secondtemperature of an underside surface of the base substantially below theposition whereat component grinding using the wheel occurs; and

[0027] (c) computing means for receiving first and second temperaturemeasurements from the first and second sensing means respectively andfor calculating therefrom a correction factor for applying to actuatingmeans for moving the wheel relative to a component to be ground, therebyreducing component size drift.

[0028] Preferably, the machine includes a coolant fluid gullysubstantially below the position in the machine whereat componentgrinding occurs, the first sensing means being spatially located withinthe gully. The inventor has found from studies that the gully is aparticularly appropriate location whereat to measure the firsttemperature in order to obtain effective thermal distortion correction.

[0029] Conveniently, for ease of installation and adjustment, the secondsensing means comprises a second temperature sensing probe magneticallyattached to the underside surface of the base.

[0030] For accurate temperature measurement in an environment whereconsiderable electrical interference is experienced, for example onaccount of the use of electronic motor control equipment, the secondsensing probe preferably includes a platinum resistance thermometer formeasuring the second temperature. Most preferably, the platinumresistance thermometer is a Pt-100 type resistance thermometer.

[0031] Preferably, the first sensing means is not directly exposed tocoolant fluid flowing on an upper surface of the base as such fluid issusceptible to transient temperature fluctuations. Thus, beneficially,the base comprises an upper panel into which the first sensing means ismounted. More preferably, the first sensing means includes a firstsensing probe which is spatially located within the upper panel and issubstantially thermally isolated from the coolant fluid flowing over theupper surface of the base.

[0032] When designing the machine, the inventor has found it beneficialto mount the first sensing means in a blind hole machined into the upperpanel. Preferably, the blind hole is prefilled with heat conductivepaste prior to installing the first sensing means into the hole. Use ofsuch paste assists to ensure that the first sensing means it not greatlyinfluenced by the ingress of coolant fluid and is also in effectivethermal communication with the upper surface of the base. The inventorhas found it especially preferable to employ a conductive pasteincluding silicone, silicone being hydrophobic and thereby repelling thecoolant fluid. Preferably, the blind hole is bored to a depthcorresponding to in a range of 60% to 90% of the thickness of the panel,and the first sensing probe is mounted substantially at the bottom ofthe hole. This range is effective at ensuring that the first probe issufficiently far from the upper surface of the base where the coolantfluid flows but not so deep that there is a risk that the hole breaksthrough into a central region of the base during base manufacture.

[0033] The first probe is preferably mechanically shielded by a carrierfor installation into the machine. The carrier is preferably a poor heatconductor, otherwise transient temperature fluctuations of the coolantfluid could influence the first probe. Thus, preferably, the first probeis mounted on the carrier for retention in the hole, the carrier beingfabricated from a substantially thermally insulating polymer. Morepreferably, the polymer includes one or more of nylon, polycarbonate,polyethylene, polypropylene and fibre-reinforced phenolic resin. Such apolymer is susceptible to being easily machined or moulded.

[0034] For ease of installation and maintenance, the carrier isconveniently retained by means of a screw thread into the hole.

[0035] In order to implement the method of reducing thermal distortionaccording to the first aspect in the machine, a reliable and accuratetemperature measurement is required. Thus, preferably, the first sensingprobe is a platinum resistance thermometer. More preferably, the firstsensing probe is a Pt-100 type platinum resistance thermometer.

[0036] The method of the invention according to the first aspect may beapplied to existing grinding machines by way of retro-fitting. Suchretro-fitting involves attaching two temperature sensors and alsoinstalling a software addition to the controlling means to enablecorrection of the grinding wheel offset during use. Preferably, thesoftware addition is operable to provide soft keys for enabling ordisabling component size drift correction according to the first aspectof the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0037] Embodiments of the invention will now be described, by way ofexample only, in which:

[0038]FIG. 1 is an illustration of a grinding machine equipped withtemperature sensing probes according to the invention;

[0039]FIG. 2 is a schematic diagram of a temperature sensing probeadapted for measuring the temperature at an upper surface of a base ofthe machine illustrated in FIG. 1; and

[0040]FIG. 3 is an illustration of a part of the machine shown in FIG.1.

[0041] Referring now to FIG. 1, a grinding machine is indicatedgenerally by 10. The machine 10 comprises a headstock 20 and a tailstock30 between which an elongate component 40 for grinding is mounted, forexample a stainless steel tubular component. The machine 10 furthercomprises a grinding wheel 50 with its associated support structureincluding one or more wheel drive motors. Moreover, the machine 10includes a control unit (not shown) for controlling its operation, theunit comprising a computer executing machine control software. The wheel50 with its associated structure, and the tailstock 30 and headstock 20are all mounted on a base 60 of the machine 10.

[0042] Operation of the machine 10 will now be described in overview.

[0043] In operation, the control unit actuates the grinding wheel 50relative to the component 40. The grinding wheel 50 is rotated andbrought into contact with the component 40 for grinding materialtherefrom and thereby machining the component 40. When necessary, thecomponent 40 is rotated about its elongate axis A-B so as to bringdifferent parts of the component 40 into contact with the wheel 50. Acoolant fluid is sprayed onto the component 40 and the wheel 50 duringgrinding, the fluid being collected into a gully 70 in the base 60 forsubsequent filtration and recirculation.

[0044] The machine 10 will now be described in further detail.

[0045] The machine 10 includes a first temperature probe 100 formeasuring a first temperature of an upper surface of the base 60, thefirst probe 100 being attached to the base 60 below a position indicatedby 110 whereat grinding of the component 40 occurs. More preferably, thefirst probe 100 is mounted in the gully 70. Moreover, the machine 10further includes a second temperature probe 120 for measuring a secondtemperature of an underside surface of the base 60, the second probe 120being attached substantially vertically below the first probe 100 andthe position 110 where grinding occurs.

[0046] Outputs from the first and second probes 100, 120 are connectedto the aforesaid control unit so that its software receives dataindicative of the temperatures of the upper and underside surfaces ofthe machine base 60.

[0047] The second probe 120 is conveniently a magnetically-retaineddevice which is simply applied to the underside surface of the base 60.The second probe 120 can alternatively be bolted or clamped intoposition on the underside surface of the base 60. However, the inventorhas found that the second probe 120 is susceptible to being influencedby transient air drafts flowing around the underside of the base 60. Inorder to desensitize the second probe 120 from such draughts, theinventor has found it beneficial to include thermal insulation aroundthe probe 120 except where the probe 120 contacts onto the undersidesurface of the base 60. Such thermal insulation can include a clothcover, an expanded-plastics foam cover or simply a plastics materialshell. Thus, the second probe 120 is preferably in intimate thermalcontact with the underside of the base 60 but shielded from airflow onthe underside of the base 60.

[0048] In devising the invention, the inventor has had to evolve thedesign of the first probe 100 to make it suitable for mounting onto theupper surface of the base 60.

[0049] Referring now to FIG. 2, there is shown the first probe 100installed into a blind hole machined into an upper exterior surface ofthe base 60. The probe 100 comprises a body section 200 fabricated froma hexagonal-top M12 Nylon bolt through which a longitudinal axial hole210 has been moulded or bored. The probe 100 further comprises atemperature sensing platinum resistance tip 220 connected to a twistedpair of wires 230. The tip 220 is located at an end region of the bolt200 remote from a hexagonal head region 240 of the bolt 200. Moreover,the wires 230 are routed from the tip 220 through the axial hole 210 tothe head region 240 wherefrom the wires 230 are further conveyed to thecontrol unit.

[0050] Although the bolt 200 is fabricated from nylon, it canalternatively be fabricated from another substantially thermallyinsulating polymer, for example polypropylene, polyethylene,fabric-reinforced phenolic resin or polycarbonate. Mixtures of thesepolymers can also be employed.

[0051] The probe 100 is mounted within a blind M12 tapped hole machinedinto the upper surface of the base 60 as illustrated. When installingthe probe 100 into the blind hole, it has been found by the inventor tobe especially desirable to prefill the hole with white heat conductivesilicone paste, for example as conventionally employed for improvingheat conductivity from power semiconductor devices such as TO3can-mounted power transistors to associated finned heatsinks. Inclusionof the paste is also effective at reducing the ingress of coolant fluidto the resistance tip 220; the silicone paste is generally hydrophobic.The base 60 is a relatively substantial grinding machine part fabricatedfrom cast iron having an upper surface panel thickness in the order of 3to 5 cm. The blind hole is preferably bored in a range of 60% to 90%through the thickness of the upper surface panel.

[0052] The inventor has found that the probe 100 is highly effective atmeasuring the temperature of the upper surface of the base 60 at thesame time as being relatively insensitive to transient temperaturefluctuations of the coolant fluid sprayed during machine operation overthe component 40 and the wheel 50. Such insensitivity is achieved byvirtue of the bolt 200 being fabricated from a substantially thermallyinsulating material. A thermally insulating material is defined as amaterial having a thermal conductivity of less than 1 W m⁻¹ K⁻¹.

[0053] When developing the invention, the inventor initially usedmagnetically-retained probes applied to both the upper exterior surfaceof the base 60 and to the underside surface of the base 60 for measuringa temperature difference therebetween. During such development, theinventor found that the magnetically-retained probed applied on theupper exterior surface was rather susceptible to transient temperaturefluctuations in the coolant fluid. Thus, the inventor has appreciatedthat mounting the first probe 100 slightly into the base 60 circumventssuch transient fluctuations, thereby rendering the method of theinvention more accurate and reliable.

[0054] The inventor found that temperature differences between the upperand lower surfaces of the base 60 can be relatively small, for exampleless than 1° C. in some grinding situations. Thus, the probes 100, 120preferably have a differential temperature measuring resolution of atleast 0.1° C. More preferable, the probes 100, 120 have a differentialtemperature measuring resolution of at least 0.05° C. Most preferably,the probes 100, 120 have a differential temperature measuring resolutionof at least 0.01° C.

[0055] When measuring small temperature differences of 0.5° C. or less,it is conventional practice to employ thermocouples connected indifferential mode. The inventor has found that such a differentialarrangement is unsatisfactory because of electrical interference in theenvironment surrounding the machine 10, for example due to high powerelectronic motor control equipment. Moreover, although thermistortemperature sensors are highly sensitive, they are generallyinsufficiently stable to render them suitable for measuring thetemperature difference between the upper surface and underside surfaceof the base 60. The inventor has therefore found that platinumresistance thermometers are most appropriate despite the need to performan accurate calibration of the probes 100, 120. The probes 100, 120preferably employ Pt-100 type platinum resistance thermometers.

[0056] The inventor has found that, despite including structuralfeatures to enhance the rigidity of the base 60, uncompensated grindingaccuracy of the machine 10 is sensitive to the temperature differencebetween the first and second probes 100, 120. In one example design ofgrinding machine, grinding errors of 6 μm were found to correlate withtemperature differences of 0.1° C. between the probes 100, 120.

[0057] Referring now to FIG. 3, there is shown a set-up of the machineshown in FIG. 1. The first probe 100 is mounted substantially verticallyabove the second probe 120, the probes 100, 120 being substantiallyco-linear along an axis C-D with the grinding region 110.

[0058] The inventor has appreciated that the core temperature of thebase top surface directly below the grinding region 110 correlatesclosely with size drift when machining the component 40. Moreover, theinventor has appreciated that the temperature of the underside of thebase 60 further improves the degree of correlation, especially when thetemperature of the base 60 converges towards the temperature of thecoolant fluid.

[0059] The inventor has further appreciated that an empiricalrelationship pertains to a diametrical machining size drift (MSD) in μmwhen grinding the component 40 and changes in first and second probetemperatures denoted by ΔT₁ and ΔT₂ respectively, the relationship asprovided in Equation 1 (Eq. 1):

MSD=K _(f)(ΔT ₁ −ΔT ₂)  Eq. 1

[0060] where

[0061] K_(f)=a proportionality constant (μm/° C.).

[0062] For one type of grinding machine modified to include the probes100, 120 as described in the foregoing, a value for the constant K_(f)in a range of 25 to 35 was found to be consistently correct; namely, theconstant K_(f) preferably has a value of substantially 30. However, thisvalue for the constant K_(f) would be expected to change if the type ofgrinding machine were changed.

[0063] In grinding machine trials applying a correction as defined byEquation 1, improvements in component batch grinding accuracy in a rangeof 63% to 81% were achieved. Such accuracy improvement was attainable inboth simulated intermittent and continuous batch grinding conditions.Moreover, the accuracy improvement was attainable irrespective ofwhether or not bedwash and wheel 50 spindle weir coolant had been leftrunning.

[0064] Applying the correction defined in Equation 1 to an operatinggrinding machine over a period of 3 days was found by the inventor toreduce component size drift from 254 μm to 69 μm.

[0065] The inventor has appreciated that a dominant effect responsiblefor grinding inaccuracy in a grinding machine is attributable to thetemperature of a coolant fluid affecting a base of the machine, and notsignificantly due to any heat generated by the grinding process itselfor from wheel wind-age. It is conventional design practice to route suchcoolant fluid through a gully formed in the base of the machine situatedbelow a region of the machine in which component grinding occurs.

[0066] In the foregoing, it will be appreciated that modifications andadditions can be made to the embodiments elucidated in the foregoingwithout departing from the scope of the invention.

[0067] For example, although Equation 1 provides a linear relationshipbetween the temperature difference between the first and second probes100, 120, it is possible for the relationship to be of a more generalpolynomial form as expressed in Equation 2 (Eq. 2):

MSD=K _(f) G(ΔT ₁ ,ΔT ₂)  Eq. 2

[0068] where

[0069] G(ΔT₁,ΔT₂)=b₁(ΔT₁−ΔT₂)+b₂(ΔT₁−ΔT₂)²+b₃(ΔT₁−ΔT₂)³+ . . .+b_(n)(ΔT₁−ΔT₂)^(n)

[0070] and where

[0071] b₁, . . . b_(n)=proportionality constants.

[0072] Such a more general polynomial form in Equation 2 includesEquation 1 within its scope by virtue of the constant b₁ being unity andthe constants b₂ to b_(n) being zero.

[0073] The function G is a polynomial function having ΔT₁ and ΔT₂ asinput parameters. Thus, the function G can, for example be a quadraticfunction, a cubic function or an even higher-order function. Moreover,the function G can also be modified to include a time parameter, forexample time from machine switch-on so that warm-up inaccuracies canalso be compensated.

[0074] The inventor has found from trials with grinding machines thatthe constant K_(f), and the constants b₁ to b_(n) can be determinedempirically from machining accuracy data collated during the trials. Thetrials are best performed with the compensation according to one or moreof Equations 1 and 2 disabled, namely with a grinding machine undertrial operating open-loop with regard to temperature compensationderived from its differential machine base temperature.

[0075] If required, data corresponding to machine base temperatures andground component metrology results, and optionally also timeinformation, can be input to curve-fitting software executing on acomputer for determining coefficients such as the constant K_(f) and theconstants b₁ to b_(n) appropriate to employ in, for example, operatingsoftware for controlling the machine 10 for improving its grindingaccuracy.

1. A method of reducing thermal distortion in a grinding machinecomprising a machine base and a grinding wheel for grinding componentsin the machine, the method including the steps of: (a) sensing a firsttemperature at an upper surface of the base substantially below aposition in the machine whereat component grinding using the wheeloccurs; (b) sensing a second temperature of an underside surface of thebase substantially below the position whereat component grinding usingthe wheel occurs; (c) determining a relationship between component sizedrift and changes in a difference between the first and secondtemperatures; and thereafter (d) correcting a positional offset appliedto the wheel during grinding in accordance with the determinedrelationship, thereby reducing the component size drift.
 2. A methodaccording to claim 1 wherein the base includes a panel at its uppersurface and the first temperature is measured within the panel away froman upper facing exterior surface of the panel susceptible to beingexposed to a coolant fluid.
 3. A method according to claim 2 wherein thefirst temperature is measured at a distance in a range of 60% to 90% ofthe thickness of the panel away from the upper facing exterior surface.4. A method according to claim 2 or 3 wherein the first temperature ismeasured in a coolant fluid gully included in the machine below theposition whereat grinding occurs, the gully being included forcollecting coolant fluid output from the position whereat grindingoccurs.
 5. A method according to any preceding claim wherein the firsttemperature is measured using a probe which is substantially thermallyisolated from any coolant fluid flowing over the upper exterior surfaceof the base.
 6. A method according to any preceding claim wherein thefirst temperature is measured using a probe which is in thermalcommunication by mediation of a heat conductive paste with the uppersurface of the base.
 7. A method according to claim 6 wherein the heatconductive paste comprises silicone.
 8. A method according to anypreceding claim wherein the second temperature is measured using a probemagnetically attached to the underside surface of the base.
 9. A methodaccording to claim 8 wherein the probe used for measuring the secondtemperature is provided with a thermally insulating shield to render itinsensitive to transient temperature fluctuations in air flowing overthe underside surface of the base.
 10. A method according to anypreceding claim wherein the position whereat component grinding occursand the positions at which the first and second temperatures aremeasured are mutually substantially co-linear.
 11. A method according toany preceding claim wherein the relationship is of the form MSD=K _(f)(b₁(ΔT₁ −ΔT ₂)+b ₂(ΔT ₁ −ΔT ₂)² + . . . +b _(n)(ΔT ₁ −ΔT ₂)^(n)) wherein:(a) MSD is a grinding correction applied; (b) K_(f), b₁ to b_(n) areproportionality constants; (c) ΔT₁ is the first temperature; (d) ΔT₂ isthe second temperature; and (e) n is a positive integer.
 12. A methodaccording to claim 1I wherein the relationship is a linear, quadratic orcubic function.
 13. A method according to claim 12 wherein therelationship is substantially a linear function of the form MSD=K_(f)(ΔT ₁ −ΔT ₂)
 14. A method according to claim 13 wherein theproportionality coefficient K_(f) is in a range of 25 to 35 μm/° C. 15.A method according to claim 14 wherein the constant K_(r) issubstantially 30 μm/° C.
 16. A method according to claim 11 wherein oneor more of the proportionality constants are calculated empirically fromopen-loop trials undertaken on the machine where temperature correctionderived from the base is not applied.
 17. A method according to anypreceding claim wherein the first and second temperatures are measuredto a resolution of at least 0.1° C.
 18. A method according to claim 17wherein the first and second temperatures are measured to a resolutionof at least 0.05° C.
 19. A method according to claim 18 wherein thefirst and second temperatures are measured to a resolution of at least0.01° C.
 20. A grinding machine employing the method according to claim1, the machine comprising a machine base and a grinding wheel forgrinding components in the machine, with the use of a coolant fluid, themachine further comprising: (a) first temperature sensing means forsensing a first temperature at an upper surface of the basesubstantially below a position in the machine whereat component grindingusing the wheel occurs; (b) second temperature sensing means for sensinga second temperature of an underside surface of the base substantiallybelow the position whereat component grinding using the wheel occurs;and (c) computing means for receiving first and second temperaturemeasurements from the first and second sensing means respectively andfor calculating therefrom a correction factor for applying to actuatingmeans for moving the wheel relative to a component to be ground, therebyreducing component size drift.
 21. A machine according to claim 20wherein the machine includes a coolant fluid gully substantially belowthe position in the machine whereat component grinding occurs, the firstsensing means being spatially located within the gully.
 22. A machineaccording to claim 20 or 21 wherein the second sensing means comprises asecond temperature sensing probe magnetically attached to the undersidesurface of the base.
 23. A machine according to claim 22 wherein thesecond sensing probe is provided with a thermally insulating shield torender it insensitive to transient temperature fluctuations in air flowover the underside surface.
 24. A machine according to claim 22 or 23wherein the second sensing probe includes a platinum resistancethermometer for measuring the second temperature.
 25. A machineaccording to claim 24 wherein the platinum resistance thermometer is aPt-100 type resistance thermometer.